London has been battered by 50mph winds that have felled trees and caused travel chaos. Powerful gusts swept across the capital as the Met Office issued a yellow "be aware" weather alert for most of the country.

London Marathon chief executive Nick Bitel is confident Sunday's race will be safe and live up to its reputation as an event at which "humanity can be united".

Bitel announced in the wake of the Boston Marathon bombings that race organisers and the Metropolitan Police had decided to employ additional security measures around the event and described the Metropolitan Police as the best in the world at handling terror threats.

Bitel said: "I am not going to get into precisely what those (security) measures are because I think that is an aid to anyone who wants to do something. I don't think they will. One of the great things about the London Marathon is that it is perceived as being this event that brings people together. One of the founding principles of the London Marathon was to show that at least on one day humanity can be united."

He added: "We are taking every reasonable step to ensure the race is as safe and secure as possible. We instigated a full security review. Considerable extra police and our own security resources will be employed.

"The Mayor made it very clear to the Commissioner (of the Metropolitan Police), and the Commissioner down to his staff as well, that it is about putting out the right number to send the right message. It is not about budget, it is not about money, it is about ensuring the police are able to do as they do every day and keep London safe and secure.

"London is one of the safest cities in the world. We saw last year with the Olympic Games the Metropolitan Police lead an incredible security effort around the Games. There is really no finer force in the world in answering these type of threats."

Some of the world's greatest marathon runners expressed their sorrow at the events in Boston - but also their determination to run without fear in London.

Kenya's Wilson Kipsang, the 2012 London Marathon winner and Olympic bronze medallist, said: "As an athlete I would like to send our condolences to those people who lost their loved ones in Boston, especially at this time they are mourning. We know they are going through a hard time."

Geoffrey Mutai ran the world's fastest time in winning the Boston Marathon in 2011 and so the events of last weekend touched him deeply. "My reaction when I saw the news about Boston, I felt very sad because I know Boston," the Kenyan said.

Olympic champion Stephen Kiprotich said his heart went out to the family of eight-year-old Martin Richard, who died as he watched the race with his family. "I feel very sorry for the people who lost their loved ones, especially the young boy who had come to witness the race. I was very sorry," the Ugandan said.